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Wind load on the tower using the drag coefficient
Hello!
I am trying to calculate the loads on the tower bottom for a later foundation and soil calculation. In the rotor part, everything looks fine, but when I enter the structural model, there are some oddities.
1) I have found significant differences between the sensors “TWR pos 0.000” and “TWR Bot. Constr”. Previously found a topic about the possible influence of attached mass on this phenomenon, but in my opinion, mass shouldn`t influence that way, at least on horizontal force (see fig.) Maybe I don’t understand something, or am I doing something wrong?
2) When I enter the drag coefficient for the tower, I get an unstable load signal with significant amplitudes (see fig.), which seems incorrect to me. Perhaps I’m using the wrong grid and time settings? I understand that I can use the loads on the top of the tower to model the “tower-foundation” system in other software, but I would like to get the whole loads on the foundation entirely from QBlade, taking into account the turbulent wind.
Thanks for your amazing work
Regards, Ivan
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Hello Ivan,
to me it looks like the simulations with tower drag activated behave quite unstable. This could be the reason why you see a difference between the two sensors for this case (the simulations without tower drag seem to be quite consistent). In theory, the shear forces at the tower bottom (TWR pos. 0.000) should be equal to the reaction forces at the tower bottom constraint. The instability could be causing this “diverging behavior”. Since QB 2.0.64 uses Euler-Benoulli beams (which don’t account for shear deformation) the shear forces are only approximated. This will change with the release of QB 2.0.7, which will also include Timoshenko beam models that explicitly include shear deformation.
When looking at your turbine design, it seems like the tower diameter is quite large compared to the size of the rotor. This could induce (depending also on the tower drag coefficient) large “jumps” in velocity, whenever the rotor pases the tower, and trigger some instability of the rotor (depending on its aerodynamic and structural properties). This could be mitigated by increasing the rotor overhang for example, so that the blades are suituated further away from the tower.
Also, since you are simulating at a windspeed of 25m/s, with a forced rotor rotation I am wondering whether you are actually pitching the blades at this operational point? This could also be a reason why the model behaves unstable, since the rotor loads could be very high in this scenario.
I would suggest to also check the structural parameters of the rotor and assess its eigenfrequencies in your analysis.
If you are interested in the tower bottom loads, to couple them with another simulation, you should use the reaction forces, given by the tower bottom constraint sensor.
BR,
David
Thank you very much, David!
Yes, I know you are preparing a release with Timoshenko beams, waiting to test in operation.
Such a big tower, because I took a wind turbine design developed by a small design office in the 90’s for which I have some examples of old calculations to compare with modeling and to study QBlade.
Since I am in the process of learning the QBlade functions in detail, in this example I do not use advanced models yet and disable all possible corrections (LLFWW, tower shadow etc.). Therefore, I assume that using pure UBEM, there should be no aerodynamic interference between the blades and the tower. It also seems that such interference should produce oscillations with frequency 3P?
Since my task is foundation design (I’m currently working on a PhD thesis about WT foundations in permafrost), I’m primarily interested in extreme states. In this case, I tried to model a rotor without pitch-control at maximum operating speed (Vcut-out = 25m/s) and 75 rpm. Besides, in the model without tower drag coefficient everything works quite normally, I observe only small harmonic oscillations with frequency 3P.
I also checked the parameters and natural frequencies of the blades in QFEM. I didn’t notice any problems or surprises, the first frequency is ~3.1 Hz, flapwise mode. Since my specialty is foundation design and geotechnics, maybe I am missing something, but it seems to me that the rotor natural frequencies should not play a big role with a uniform wind…
I will try to set the wind load on the tower using an external loading file, I have already started writing a Python script to create such file using TurbSim windfield. I hope to get a more stable result.
Regards, Ivan
